There are numerous situations in which the location of an aircraft's center of gravity has an effect on the manner in which the aircraft control surfaces are operated to maneuver and navigate the aircraft. Thus, in automatic flight control systems it is necessary to at least estimate aircraft center of gravity in order to generate proper command signals. Since no onboard equipment exists for determining the center of gravity, the practice has been to use a worst case estimate of the center of gravity in automatic flight control situations in which center of gravity is an important parameter. Although the practice of using a worst case estimate provides generally satisfactory results, less than optimal control action may result and certain disadvantages and drawbacks may arise.
One situation in which location of aircraft center of gravity is important in automatic flight control is the use of stabilizer mistrim during the final stages of an automatic landing procedure. As is known in the art, stabilizer mistrim is utilized in automatic landing procedures to cause a flare-like maneuver in the event of an automatic landing system disconnect caused by a system failure or by the pilot taking manual control of the aircraft. Specifically, during final approach, the automatic landing system drives the stabilizer in a nose-up direction by supplying a command signal to the elevator trim. Opposite elevator is commanded by the system as the stabilizer moves to cancel the pitching moment produced by the stabilizer nose-up command signal. In the event of automatic landing system disconnect, a pitch-up moment is generated as the elevator moves to its neutral position. Thus, the rate of descent (sink rate) is slowed and the pilot is given adequate time to assume control and execute a relatively normal landing (e.g., without an excessively high sink rate or the aircraft nose gear contacting the runway before the main gear).
Optimal stabilizer mistrim primarily depends on the aircraft approach speed, the flap setting being used during the landing procedure and the location of the aircraft's center of gravity. Prior to the advent of this invention, only two options were available for applying stabilizer mistrim during automatic landing procedures. First, the automatic landing system could employ a fixed or predetermined amount of stabilizer mistrim based upon information gathered during flight tests and/or an estimated worst case aircraft configuration and approach speed. Alternatively, a worst case center of gravity location would be assumed and signals representative of the aircraft approach speed and flap setting could be processed to provide stabilizer mistrim that is at least partially compensated for the actual landing profile of the aircraft.
Worst case center of gravity location for automatic landing stabilizer mistrim occurs with a heavily loaded aircraft and the center of gravity located at its most forward position. However, establishing stabilizer mistrim for worst case center of gravity location can result in more than the desired nose-up pitching moment should the automatic landing system be disengaged during the landing procedure. For example, with respect to one particular aircraft and automatic landing system, it was determined that the amount of stabilizer mistrim required for a lightly loaded airplane with its center of gravity at the aftmost position was 34% less elevator than was required with a heavily loaded airplane in which the center of gravity was at the forwardmost position. Establishing stabilizer mistrim based on the worst case center of gravity situation (aircraft heavily loaded/center of gravity forward), thus meant that a fairly significant pitch-up attitude could result if the automatic landing system was disengaged while the aircraft was lightly loaded and the center of gravity was located at its aftmost position. With this particular system, the situation could be further complicated if the automatic landing system was disconnected prior to the point at which the aircraft automatic throttle system was operated to retard engine throttle settings. In that situation, if the pilot did not immediately manually retard the throttle settings, the throttle could be further advanced by the aircraft autothrottle system and additional nose-up attitude would result.
Although establishing stabilizer mistrim on the basis of worst case expected center of gravity location achieves the goal of allowing adequate time for the pilot to assume manual control and execute a safe landing, the more than sufficient pitch-up attitude that results when the aircraft is lightly loaded and its center of gravity is located nearer the aftmost position is at least somewhat undesired. In this regard, placing the aircraft in a nose-up attitude that is more than sufficient for the assumption of manual control and safe landing can result in a longer than normal landing distance. That is, because of the time required for the pilot to restore the aircraft to a relatively normal landing profile, the aircraft will not touch down on the runway as early as it would had less stabilizer mistrim been applied by the automatic landing system. Under extreme conditions, it may be necessary for the pilot to apply fairly substantial nose-down manual control and, in some cases, execute other fairly aggressive maneuvers in order to quickly bring the aircraft into the desired landing profile.
A need for even occasional substantial nose-down manual command and/or other relatively aggressive manual control of the aircraft gives rise to two relatively important operational considerations of an automatic landing system. First, it is important that pilots that operate aircraft equipped with an automatic landing system have a high degree of confidence in the system. This confidence is necessary both to obtain certification of a system by regulatory authorities and, in addition, to insure that pilots do not prematurely disengage the system when adverse conditions are encountered. Some automatic landing systems utilizing worst case center of gravity estimation to establish stabilizer mistrim could, in the event of system disconnect, produce excessive nose-up pitching moment under lightly loaded and aft center of gravity conditions that would cause concern on the part of at least some of the pilots flying aircraft equipped with the system. The second consideration is that of passenger comfort and confidence. Specifically, even though an aircraft is operating well within safe bounds, passengers can become apprehensive, concerned and uncomfortable during somewhat aggressive maneuvering of the airplane. Thus, using worst case center of gravity conditions to establish stabilizer mistrim in an automatic landing system can result in unnecessary passenger concern and discomfort should the automatic landing system be disengaged and manual pilot control be asserted during an automatic landing procedure in which the aircraft is lightly loaded and the center of gravity is located in an aft position.
For the above-discussed reasons there is a need for an arrangement that provides real time estimation of the aircraft's center of gravity to thereby allow stabilizer mistrim that is adapted to or compensated for actual landing conditions. Further it will be recognized by those skilled in the art that there are numerous other situations in which aircraft center of gravity is used in establishing open loop compensation or feedback gains control laws that are used by automatic flight control systems to navigate and control an aircraft. In many cases, performance of such systems also can be improved by utilizing a real time center of gravity estimate instead of a predetermined, worst case estimate.